Heterochromatin is a tightly packaged form of DNA that is associated with transcriptional repression. It is essential for normal development and genome stability. Abnormal heterochromatin is implicated in the pathology of the human developmental syndrome Immunodeficiency, Centromere region instability and Facial abnormalities (ICF), the degenerative muscle wasting disease Facioscapulohumeral muscular dystrophy (FSHD) and is likely to facilitate aneuploidy in cancer. Heterochromatin is required for a number of essential biological processes including normal chromosome segregation, sex chromosome dosage compensation, monoallelic expression of imprinted genes and transcriptional silencing of parasitic DNA elements. To date, much of our molecular understanding of heterochromatin regulation is derived from genetic screens performed in plants, insects and fungi. Mouse mutations in genes identified in screens from these organisms demonstrate conservation of many aspects of heterochromatin regulation, including critical roles for histone 3 lysine 9 methylation and the heterochromatin protein HP1. However, there are differences between heterochromatin regulation in vertebrates and other model organisms. For example, the DNA modification 5-methylcytosine is an essential component of heterochromatin in vertebrate species but is not present in the genomes of Schizosaccharomyces pombe or Drosophila melanogaster. Zebrafish offer a powerful combination of genetic, molecular and developmental tools that allow for innovative approaches to the study of vertebrate heterochromatin. This proposal uses these tools to define molecular pathways that regulate heterochromatin at repetitive sequences and to identify requirements for the involved genes in normal development and tissue homeostasis. In a small screen for regulators of zebrafish heterochromatin, our laboratory identified several genes including zbtb24 and nsd1a. ZBTB24 is mutated in humans with ICF syndrome type 2 and NSD1 is mutated in the human developmental disorder Sotos syndrome. However, roles for the encoded proteins in heterochromatin regulation have not been experimentally addressed. The proposed research will define endogenous sites of heterochromatin that depend on these factors and identify the molecular functions of these proteins in heterochromatin regulation. Based on early success, more extensive candidate and unbiased loss-of-function screens will be performed to identify and define the molecular functions of additional genes that regulate heterochromatin in the zebrafish embryo. Elucidation of the molecular and developmental requirements for these genes is expected to uncover novel mechanisms for heterochromatin regulation that are conserved in vertebrate species and which are relevant to human development and disease states.